McCollough effect: what is this illusory phenomenon?

Optical illusions can be very curious. Generally, they occur when we are exposed to very specific patterns of shape and/or color for a while; altering the conventional way in which we perceive lines, curves and even more complex stimuli (the human face, for example).

Illusions are also phenomena that are expressed in completely healthy people, and that generally revert in just a few seconds. This is precisely what distinguishes them from hallucinations, which usually persist and are not derived from the objects that are within the perceptual framework.

In this article we will address what is one of the most relevant optical illusions in the history of science, the McCollough effect, whose particularities have surprised the entire research community during decades. In fact, we still do not have a theoretical model that can fully explain it.

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What is the McCollough effect?

The McCollough effect was discovered in 1965 by Celeste McCollough, a psychologist who was already studying other phenomena. perceptual in the first decades of the last century, although it was defined more exhaustively over the years subsequent

It is an optical illusion included in the post-effect category, that is, afterimages that require a prior period of exposure to a specific pattern of stimuli in order to unfold. In these cases, very specific colors or shapes are usually used, which only affect the most superficial aspects of visual processing and last a few seconds.

The effect that concerns us, however, is somewhat more complex and has been considered as a mechanism potentially explanatory for the way in which visual stimuli are usually integrated at the level central. That is why for many years, since it was first described, it has motivated research varied in which the original methodology has been changing in order to explore the exact origin of the freak. Despite everything, there is still a lack of definitive knowledge on this matter, although there are some clues that guide where to continue looking.

Next we will see how to carry out the procedureor, what are the "effects" that can be foreseen behind it and the mechanisms that are at its base. However, it is essential to point out at this point that we are not dealing with a game, but with a methodology that promotes changes in brain structures and that it can be maintained for a long time (depending on how it is carried out). cape). Everything that is described below comes from the scientific heritage on this issue and it is interesting to know it, but it should not be carried out without the corresponding information and always under the entire responsibility of the person who so decide.

Procedure

The McCollough effect (like the rest of the illusions in its category) requires a prior induction stage, in which the person must be exposed to two colored grids alternately. Specifically, it is a pattern of red horizontal lines (on a black background) and another of green vertical lines (with the same background). Both are shown to the subject for about three seconds, oscillating from one to the other for a period of time that generally lasts three minutes (although it may vary according to the intention of the person conducting the study).

After this adaptation period, the person is shown a figure made up of black/white lines, arranged both horizontally and vertically. It is a complex grid that includes the patterns described above, but at this stage it lacks all chromaticism (it only preserves the direction of the lines). This new stimulus is presented to the subject in a contingent manner. (at the end of the induction phase), and the first evidence of perceptual alteration is manifested on it. Sometimes this monochrome figure is also shown before the induction, so that it is appreciated that it really lacks colors and the effect is more evident.

the illusionary effect

Upon exposure to the colored grids, the subject will observe that the originally monochrome grid will take on different hues in the white spaces. More specifically, it will be appreciated that the horizontal ones will acquire a greenish tone and the vertical ones reddish/pink. That is, the "inverses" to those that were shown during the previous induction period (their negatives). So far it is nothing too striking or new, since there is a great variety of "tricks" to reproduce the same effect, but in the one in question would have a particularity: it can last many days, up to three and a half months in the case of very long periods of induction.

The effect has also been reported with lines of different colors, such as blue and orange., with results whose duration is directly related to the induction time. Thus, it has been tested with periods of just a few seconds and up to 150 minutes, being in the latter cases a greater persistence of the afterimage. In any case, there were many people who used computer monitors (green phosphor) in the decades of the 60-80 that They were able to offer pioneering testimony on this matter, since they reported reading books with a soft pink or red background.

In addition, it has been proven that the intensity of the post-image colors is also linked to the total time of previous exposure, in such a way that those who As soon as they observe the original green/red patterns for a few minutes, they will only be able to distinguish pale negatives of both (light greenish and pink tones), but those who are exposed for ten minutes or more will appreciate them in a much more vivid and clear way. Thus, both the intensity and the duration will depend closely on the previous period of induction.

Another very curious fact about the McCollough effect is that it produces what is known as a transfer interocular: even if the test is only carried out using one eye, its consequences extend to both. Many of our readers may be able to evoke the experience of exposing their eyes (unintentionally) to different levels of light, in such a way that one of them perceived the darkened tones and the other more clear. In such a case there would be no interocular transfer, since the effect is found in the retinal cells related to light perception (rods/cones), but then... What happens in the case at hand? Let's keep investigating.

Why happens?

Many different theories have been put forward over the years to explain why the McCollough effect occurs, but we still have only partial knowledge on the matter. The first hypotheses were based on the possibility that it was a phenomenon based on the principles of classical or Pavlovian learning (remodeling of the nervous system based on the continuous presentation of the stimulus), but it was dismissed due to the fact that it only occurred with linear figures, and not with curves or other forms of greater complexity.

The second hypotheses were related to the activity of retinal cells associated with color.: the cones, since they have photopigments for blue (cyanolabe), red (erythrolabe) and green (chlorolabe); that tend to decay due to the chromatic antagonism resulting from simple overexposure. This is what usually happens during a conventional afterimage illusion. However, in these cases the change is only maintained for a few seconds (a few minutes at most), and is never gives a transmission of this to the two eyes, so it's a line that was discarded for the effect McCollough.

On the other hand, it is evident that in a natural environment the green/red linear pattern that forms the stimulus that triggers this illusion can rarely be appreciated, so it is possible that the brain interprets it as a kind of sensory aberration and promote mechanisms to "compensate" it during the processing of visual information. In this case, for its explanation we should resort to the substratum of the central nervous system, ignoring the learning and sensory theses. Is it possible that the answer lies precisely in this mysterious organ?

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The brain and the McCollough effect

When we are exposed to a stimulus for a long time, the brain stops trying to give it attention and simply "understands" that this is so, starting to "ignore" it and leaving its resources available to perceive the rest of the things in the world. around. The same thing may be happening to you right now if you are reading this article from a mobile phone: despite the fact that you hold it with one hand, your brain is isolating everything accessory from the experience (their sense of weight, for example), and only makes an effort to understand the text. Well, a similar phenomenon happens with the illusion that concerns us.

When the eyes are constantly exposed to green/red lines, the brain comes to understand that this pattern (very rare in nature) will always be so in any of the situations possible. For this reason, will anticipate it in the presence of stimuli that harbor a relationship of similarity with it, such as horizontal and/or vertical monochrome lines. This will also occur during the first phases that follow one another as part of visual processing, but beyond what happens before reaching the retina (due to the aforementioned transfer effect interocular).

Thus, who is hogging the spotlight in recent years is the primary visual cortex, which is located in the posterior region of the brain parenchyma (occipital lobe). This zone (V1) is specialized in the perception of static and moving objects, but above all in the pattern recognition (such as those that occur during the induction phase of the McCollough effect). Likewise, it is also the point where the images of both eyes merge, forming integrated and coherent scenes (binoculars).

The hypothesis that is currently being considered the most involves alterations in this area, basic to understanding the way in which we represent colors and shapes at a cortical level. Despite this, they are still uncorroborated models, heuristics that serve to guide research activity. (Based on neuroimaging techniques and comparative studies that include subjects with very severe brain lesions). various).

Despite the fact that the aforementioned effect tends to fade as time goes by, there is also a supposed method to stop it. In such a case, new grids would be presented (but with their color toned down) to help the brain relearn that the previous pattern is no longer valid (and recover a "normalized" perception). The McCollough effect is considered a method of "modifying" brain structure through exposure to a image, and despite the fact that its effect is not permanent, it should not be carried out without precise knowledge of what it is and its scope.

Bibliographic references:

• Ans, B., Marendaz, C., Herault, J. and Seré, B. (2010). McCollough Effect: a Neural Network Model based on Source Separation. Visual Cognition, 1(6), 823-841.
• Ramachandran, V. and Zeve, M. (2017) Synesthesia and McCollough Effect. i-Perception, 8(3), 201-211.